Good morning, as per your complaint I would sugggest it is most probably a case of relapse of infection in the root canal treated tooth. It can also be because of adjacent tooth infections.
A perfectly root canal treated tooth does not have pain. Additionally because of the missing cap, the tooth might have fractured or may fracture in the future.
Next Steps
Take an anti inflammatory for pain relief, immediately get an X-ray done to check for periapical abscess.
Kindly get a crown on the rct treated tooth